Chinese energy has the features of abundant coal, poor oil and deficient natural gas. Coal plays a dominant role in Chinese primary energy consumption structure. Compared with the techniques of coal-to-methanol, coal-to-dimethyl ether, and coal-to-liquid, etc., coal-to-synthetic natural gas (SNG) has a high energy efficiency (more than 60%), consumes less water by calorific value per unit, discharges less CO2, has high utilization efficiency of waste heat (high temperature and high pressure steam is produced as by-product). Based on the above reasons, if the coal is used as starting material to produce the alternative natural gas by methanation reaction, the heat value of natural gas is greatly increased while the safety of transport and use thereof is improved, and the air pollution caused by the traditional coal combustion may be reduced. It meets the development direction about clean and high efficiency utilization of the coal. Meanwhile, the coal energy is clean and may be used in industry and civilian and supports the efficient use of the resourceful coal in the Chinese remote area like Xinjiang Uygur Autonomous Region and Inner Mongolia after the produced SNG is transported via the existing natural gas pipelines and go through pressure adjustment and gas mixture.
Coal-to-SNG process typically combines the coal gasification and syngas methanation. The catalytic syngas methanation is a strong exothermic reaction. Over a proper catalyst, CO could be completed converted under the optimized operation conditions. In order to maximally increase producing efficiency and operation continuity of methanation reaction device, it is the key point of fast removing way design of the heat discharged in methanation process. According to the different methods of heat removal, syngas methanation technologies could be divided into two types:
One type is sharing heat load through a plurality of adiabatic fixed-bed reactor connected in series and reducing gas temperature of reactor outlet by product gas circulation or multistage raw material gas. The representative technology includes Lurgi, TREMP™ and CRG technology, etc., which can separate mathanation reaction process from exchange heat process. To realize the separation, it needs to use many reactors and heat exchangers. Thus it increases equipment investment and has a low tolerance to raw material loads change.
The other type is methanation process integrating reaction and heat exchange. In 1970s, Germany Linda company developed a new technology of producing natural gas from coal, wherein a methanation main reactor is designed as an isothermal fixed-bed reactor, a heat exchanging pipe is installed inside a catalyst bed layer in order to remove reaction heat to produce high pressure steam, and thus the reaction is carried out under isothermal condition. However, as the heat exchanger is installed inside a fixed-bed catalyst bed layer, the catalyst particles have a higher heat transfer resistance and local overheating will occur when reaction is strong or space velocity of reaction is relatively high, which results deactivation of the catalyst. Therefore, the production efficiency is restricted greatly.
Moreover, CN101817716A discloses a new method and a device for catalytic methanation of synthesis gas. Firstly the synthesis gas realize the conversion rate of raw gas from 60% to 95% in a fluidized bed reactor. Secondly the synthesis gas enters into a fixed-bed reactor and then the final conversion rate reaches over 98% via reaction. A heat exchange pipe is installed in the fluidized bed reactor in order to remove the heat generated by the reaction and produce high quality steam as by-product. In term of the process, the method is simpler than the fixed-bed multistage adiabat reaction process. The fluidized bed reactor can realize isothermal operation to improve the space-time yield of methane. However, the catalyst particles are easy to be corroded in the fluidized bed. The catalyst particles below 200 μm have a lower operation gas velocity, which need larger volume of the reactor (especially, the reactor with the heat exchange pipe installed inside the reactor). Meanwhile a dense phase fluidized bed reactor with build-in heat exchange pipe has a larger pressure drop. Therefore, the process of fluidized-bed complete methanation is still under development in the laboratory and needs further improvement.
CN102180756A discloses a method for direct methanation of synthesis gas using a circulating fluidized bed, wherein the catalyst is fixed in the gas-solid-solid reactor. this process includes the following steps: the raw gas with inert heat carrier particles (alumina microsphere, quartz sand, florin ore sand) undergo complete methanation reaction in the gas-solid-solid reactor, the production gas and the inert heat carrier particles are separated by a cyclone separator, and the inert heat carrier particles are recirculated to the reactor. This method would sharply reduce reaction temperature and reduce carbon deposition of the catalyst.
Compared with method disclosed by CN102180756A, CN102180757A discloses a method in which the catalyst is coated on the surface of the grid inner wall of the reactor, and the inert heat carrier is replaced by the CO2 absorbent (CaO/MgO). Both two methods fix the catalyst in the reactor, which inevitably corrode the catalyst because of the flow and collision between the gas and the inert heat carrier or absorbent. It is difficult to recirculate the catalyst particles and the inert heat carrier because of mixing of the catalyst particles and the inert heat carrier.
JP60163828A discloses a recirculating fluidized bed device (FIGS. 5 to 7). The device is comprised of a riser 11, a dense phase fluidized bed 12, an inner cyclone separator 13, and a heat exchanger 14 or a cooling fluidized bed 15. After the synthesis gas undergoes the catalytic methanation reaction in the riser reactor, the synthesis gas and the catalyst particles concurrent flow into the dense phase fluidized bed, and then are separated into the gas and the solid by the inside cyclone separator. The catalyst particles recirculates back to the riser after being cooled via the heat exchanger or the cooling fluidized bed. Therefore, the pressure drop of the whole process (FIG. 8) isΔP=ΔP1+ΔP1′+ΔP2+ΔP3
Wherein
ΔP1 is the pressure drop of the riser during the reaction,
ΔP1′ is the pressure drop of the dense phase fluidized bed,
ΔP2 is the pressure drop of the cyclone,
ΔP3 is the pressure drop of catalyst recycling process.
Due to the pressure drop ΔP1′ of the dense phase fluidized bed layer is relatively large, it consumes too much power. As methanation is a volume-decreasing reaction, no expanding portion would be required to reduce the catalyst entrainment.
Therefore, the process for catalytic methanation of synthesis gas is further needed to be improved.